text: Lipids are a broad category of mid-sized molecules that are hydrophobic or amphipathic.  In surfactant, two subcategories of lipids are relevant: phospholipids and sterols.  Sterols are represented by cholesterol, which has an important role in the overall structure and motion of the lipids as a whole, but is vastly outnumbered by the phospholipids in surfactant.
  DPPC (dipalmitoylphosphatidylcholine), as mentioned above, is a lipid with very useful stabilizing and compacting attributes.  SP-B works primarily with this lipid, and moves it to the gas/fluid interface where it minimized surface tension. Essentially, DPPC is so important for lung function because it can shrink or expand to fit the space necessary, and a continually shrinking and expanding lung requires components like this.
  Other lipids found commonly in surfactant include phosphatidylglycerol (PG), phosphatidylinositol (PI), phosphatidylethanolamine (PE), and phosphatidylserine (PS).